Molded plastic packages are widely used to house microelectronic devices, such as silicon based semiconductor integrated circuits. The molded plastic packages are characterized by low cost, ease of assembly and adequate protection of the device from water vapor and other sources of corrosion. Usually, the device is mounted on a centrally positioned die attach paddle. The electronic device is then electrically interconnected to the inner ends of a plurality of leads which approach the die attach paddle from at least one side and up to all four sides of the paddle. Electrical interconnection is typically by wire bonding or tape automated bonding. Following electrical interconnection, the centrally positioned die attach paddle, electronic device and inner portion of the leadframe are encapsulated in a molding resin by a process such as transfer molding. The resin forms a hard, relatively moisture impervious shell protecting both the semiconductor device and the electrical connections.
Subsequent to encapsulation, the outer leads of the leadframe are frequently soldered to a printed circuit board or other external electrical device. During soldering, the temperature of the encapsulated package may rise to from about 200.degree. C. to about 260.degree. C. Particularly susceptible to temperature increases are leadless chip carriers such as plastic surface mount components. Any water vapor trapped within the package rapidly expands. One site for the accumulation of water is at the interface between the molding resin and the die attach paddle. When the accumulated water vapor expands, the base of the molded plastic package is deformed leading to what is known as the "popcorn effect".
When the popcorn effect is severe, the molding resin can fracture. The fractures form conduits allowing water vapor and other corrosives to accumulate in close proximity to the bonding wires and integrated circuit device. Water enters the package by many means. Exposure of the molding resin to moisture either before or after encapsulation can lead to a build-up of moisture within the package. The Institute for Interconnecting and Packaging Electronic Circuits (IPC) has recently proposed shipping plastic surface mount components in hermetic containers to prevent the build-up of moisture.
A second source of moisture is penetration along metal/plastic interfaces. The adhesion between the molding resin and the metallic leadframe is primarily mechanical in nature. Frequently, a gap exists between the metallic leadframe and the plastic encapsulant. Moisture travels through the gap and accumulates under the die attach paddle.
Various means have been proposed to limit the ingress of water vapor. U.S. Pat. No. 4,866,506 to Nambu et al. discloses forming a vent hole in the molding resin. The vent hole interconnects the backside of the die attach paddle with the atmosphere so moisture accumulating under the paddle will dissipate when heated. U.S. Pat. No. 4,855,807 to Yamaji et al discloses vent holes located along the tie bars which support the die attach paddle.
To minimize separation between the molding resin and the metallic leadframe, several means to improve the adhesion have been proposed. These solutions include both means to increase mechanical adhesion and chemical adhesion. To improve mechanical locking, various configurations of holes, grooves and hemispheres have been formed in both the leads and the die attach paddle. The holes and deformations increase the surface area of the leadframe component and also provide crevices for enhanced mechanical locking. For example, U.S. Pat. No. 4,862,246 to Masuda et al discloses forming a series of hemispherical depressions on the die attach paddle. These depressions increase the adhesion of the die attach paddle to the molding resin increasing the resistance to humidity.
A dull layer of nickel applied to a copper alloy leadframe has been found to increase the strength of an epoxy bond as disclosed in U.S. Pat. No. 4,888,449 to Crane et al. U.S. Pat. No. 4,707,724 to Suzuki et al. discloses coating the die attach paddle with an alloy of tin/nickel or iron/nickel will increase the adhesive strength and minimize peeling.
Certain chemical solutions also increase the adhesive strength of the bond between copper and a polymer. U.S. Pat. No. 4,428,987 to Bell et al discloses pretreating the copper surface to improve adhesion. The surface is electrolytically reduced and then coated with a solution such as benzotriazole.
While the prior art processes are effective to increase the adhesion between a molding resin and metal leadframe, the bond is still inadequate. Channels permit the ingress of water vapor. Absent complete sealing of the channels, popcorning is still a problem.